Both CDK8 (Cyclin-Dependent Kinase 8) and its regulatory partner CycC (Cyclin C) are dysregulated in a variety of human cancers. CDK8 has been recently identified as an oncoprotein in melanoma and colorectal cancers, and CDK8 is amplified and/or overexpressed in more than half of these cancers. Depletion of CDK8 effectively blocks the growth of melanoma and colorectal cancer cells, which underscores the centrality of CDK8 in gene expression and demonstrates why CDK8 is considered an attractive and promising target for cancer treatment. However, how dysregulation of CDK8-CycC contributes to tumorigenesis, which is obviously of public health importance, remains poorly understood. To determine the normal and dysregulated functions of CDK8, it is essential to identify both the upstream regulators and the downstream effectors of CDK8-CycC. Unfortunately, analyses of function and regulation of CDK8 have been hampered by the lack of specific phenotypes or in vivo readouts for CDK8 activity in multicellular organisms. Because CDK8 and CycC are highly conserved in eukaryotes, we have solved this challenge by using Drosophila, which provides unparalleled sophistication in manipulating CDK8 activity in vivo. The long-term goal of our research is to elucidate the function and regulation of CDK8. Using Drosophila as an experimental system, we have carried out an unbiased genetic screen to identify important upstream regulators and downstream effectors of CDK8. We have identified ~70 enhancers and suppressors of the CDK8/CycC-specific phenotypes. Several candidate modifiers of these phenotypes are components of the EGFR (epidermal growth factor receptor) signaling pathway, dysregulation of which is well established in tumorigenesis of various human cancers. However, the potential link between CDK8 and EGFR signaling is novel and unexplored. Thus, the objective of this proposal is to further identify the specific mutant genes from our initial modifier genetic screen and as an example to illustrate our general approach to understand CDK8 in future studies, we will also determine how CDK8 interacts with the EGFR signaling pathway. By identifying additional genes whose products function either upstream or downstream of CDK8, our genetic analyses will enable us to elucidate the regulatory network of CDK8. In addition, this study will define the molecular mechanisms for functions of CDK8 in interacting with the EGFR signaling. These results will open up many new research directions, which will significantly advance our understanding of how dys- regulation of CDK8-CycC contributes to tumorigenesis. Because CDK8 and CycC are well conserved in eukaryotes, the genetic interactions and the underlying molecular mechanisms that we identify in Drosophila will provide a working model in human studies, and will significantly impact how we evaluate the efficacy of targeting CDK8 in treating human cancers in the future.